I also encounter this issue on our installatio with: v4.4.3.7 Doc server, v9.1.0.418 COM server which is running with enterprise license on it.

At the moment users do periodically encounter this issue and are getting quite annoyed by it. I think it even led to data loss first few times, before this behaviour was noticed.

Over all some work around should be made for this as typical office workers will leave tabs with opened doc's in browsers for a long time and admin can't really follow up on that. Without fix it kind of breaks idea of fast editing mode.

So far my idea for mitigating this is to set site cookie life time to some smaller value so that users get logged out after a while to avoid documents to be opened for to long, but's that's only a temporary solution to the problem. And I'm not event that sure if it's the best approach to this.